1. Field of the Invention
The present invention relates to a ram head for self-driven pneumatic ram drills for producing boreholes in the earth, having a generally tubular housing within which there is movable axially back and forth an impact piston whose impact energy can be transferred to the ram head which is arranged at the front end of the housing.
2. Description of the Prior Art
Self-driven pneumatic ram drills are known in various embodiments and with different developments of the ram head. Aside from ogival ram heads, there are slightly conical ram heads with flat or possibly slightly recessed front surface. Finally ram heads are known which are displaceable freely or under spring action by a certain amount with respect to the housing of the ram drill.
Ram heads which are, as a whole, longitudinally displaceable with respect to the housing are intended to make certain that obstacles in the soil are crushed more easily since the impact energy of the impact piston acts only on the mass of the ram head, which is small as compared with the entire ram drill, so that the large mass of the housing is moved axially forward only after the obstacle has been already split. This manner of operation applies, however, only for certain types of soil. Particularly in the case of finely granular and compactly stratified sandy soils a ram head which is longitudinally displaceable with respect to the housing produces the result that the soil is initially compacted even more by the impact momentum in front of the ram drill and that thereafter the subsoil is merely elastically deformed. In these cases, the ram head moves forward under the action of the impact momentum of the impact piston and then back again under the energy of resilience of the soil without the actual ram drill achieving any axial advance.
Completely different behavior is obtained under these limiting conditions with a ram drill whose ram head is firmly connected to the housing. Due to the conical development of the ram head, the device wedges itself forward, the lateral wedge force displacing the earth sidewards in front of the ram head without excessive compacting. The wall friction on the periphery of the housing of the ram drill prevents the housing from being pushed back and forth axially under the action of the slight forces of resiliency. The disadvantage of these ram heads which are firmly connected with the housing as compared with ram drills with longitudinally displaceable ram heads is not only the smaller crushing power but, in particular, the very slight stability in travel in a non-homogeneous subsoil.
Aside from their use for producing holes in the earth, the ram drills have another desirable possibility of use as a ram. In this case the ram head of the ram drill transmits the impact momentum via a suitable transition piece into a pipe to be driven and drives it into the soil. A ram drill with longitudinally displaceable impact point is not well suited for this use since the ram head is under the constant danger of breaking off and converts a considerable part of the introduced ramming energy into deformations and transverse oscillations. Furthermore, a longitudinally displaceable ram head cannot be axially connected with a pipe by a self-locking taper connection, while a ram head which is firmly connected with the housing and has an outer taper, for instance of a slope of 1:10, can be readily driven from the rear into a pipe of suitable diameter by ramming strokes, a connection which is resistant to axial thrust between the ram head and the pipe to be driven being produced by the taper wedge force. Only such a connection affords the possibility of connecting a ram drill firmly to a pipe to be driven, particularly a steel pipe, so that the recoil force of the compressed air which moves the impact piston within the ram drill can no longer loosen the connection made.
The object of the present invention is to develop a ram head for self-driven pneumatic ram drills in such a manner, while avoiding the disadvantages of the known prior art, that while on the one hand it splits obstacles present in the earth with high energy and crushes them, on the other hand it avoids excessive compacting of small grains of earth in front of the ram drill and, in addition, makes it possible to drive pipes with an axially force-locked connection.